This invention relates to a laser beam machining apparatus and, in particular, to a laser beam machining apparatus adapted to form a fine via hole in a printed wiring board.
In recent years, a portable electronic apparatus such as a digital camera, a video camera, and a mobile telephone set becomes more and more compact in size and superior in function. As a consequence, a printed wiring board contained in the electronic apparatus is improved so as to increase the density of various components mounted thereon and to reduce the lead pitch. In order to cope with such improvement, a via hole formed in the printed wiring board is required to have a diameter not greater than 0.3 mm.
Heretofore, a boring operation to form the via hole in the printed wiring board is carried out by tool machining using an NC (Numerical Control) drill or light exposure machining (photovia technique). However, the NC drill can not form the via hole smaller than 0.2 mm and is often broken. On the other hand, the photovia technique can not form the via hole smaller than 0.15 mm and requires a high material cost for light exposure.
In order to solve the above-mentioned disadvantages, proposal has recently been made of a laser beam machining apparatus for boring the via hole in the printed wiring board by the use of a laser beam. The laser beam machining apparatus comprises a laser oscillator for producing a pulsed laser beam. The laser beam machining apparatus carries out the boring operation to a desired depth by adjusting the number of laser pulses per via hole or the laser energy per pulse. On the other hand, in order to obtain a desired diameter of the via hole, a mask is arranged in an optical path of the laser beam to define the diameter of the laser beam. By reducing the diameter of the laser beam with the mask, the diameter of the via hole is reduced.
Furthermore, machining by the laser beam gives no damage to a metal. Thus, the laser beam machining apparatus is advantageous in that the boring operation is performed without damaging a conductor pattern formed on the printed wiring board.
The laser oscillator typically comprises an excimer laser. However, the excimer laser has a relatively low working speed and therefore requires a high running cost because of its low etch rate (boring depth per pulse). In contrast, attention is directed to a TEA (Transversely Excited Atmospheric Pressure) CO.sub.2 laser having a narrow pulse width, a high peak power, and a high energy density. If the laser beam machining apparatus uses the TEA CO.sub.2 laser as the laser oscillator, the etch rate is as high as ten times or more as compared with the excimer laser. It is therefore possible to reduce the number of laser pulses required to form each via hole and to increase the working speed.
However, even if the above-mentioned TEA CO.sub.2 laser is used, the working speed is restricted. It is therefore desired to achieve an improvement enabling reduction in working cost per via hole.